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Conversion of C5 carbohydrates into furfural catalyzed by SO3H-functionalized ionic liquid in renewable #-valerolactone Haizhou Lin, Jingping Chen, Yuan Zhao, and Shurong Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01975 • Publication Date (Web): 06 Mar 2017 Downloaded from http://pubs.acs.org on March 6, 2017

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Title page

Conversion of C5 carbohydrates into furfural catalyzed by SO3H-functionalized ionic liquid in renewable γ-valerolactone

Haizhou Lin, Jingping Chen, Yuan Zhao, Shurong Wang﹡

State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China

﹡Corresponding author (Shurong Wang) Postal address: State Key Laboratory of Clean Energy Utilization, Zhejiang University Zheda Road 38, Hangzhou 310027, China Tel: +86 571 87952801 Fax: +86 571 87951616 Email address: [email protected]

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Graphical abstract:

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Highlights 1. An efficient and green reaction system was proposed for furfural production 2. A high FF yield of 78.12% was obtained from xylose 3. SO3H-functionalized ionic liquids showed good performance in furfural production 4. GVL could improve the catalytic activity and suppress the side reactions.

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Conversion of C5 carbohydrates into furfural catalyzed by SO3H-functionalized ionic liquid in renewable γ-valerolactone Haizhou Lin, Jingping Chen, Yuan Zhao, Shurong Wang* State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China Abstract: To obtain an efficient and green reaction system for the production of furfural (FF), this study investigated the use of a SO3H-functionalized ionic liquid as a catalyst for the conversion of xylose in a solvent of renewable γ-valerolactone (GVL). A high FF yield of 78.12% was obtained in this reaction system catalyzed by 1-propylsulfonic-3-methylimidazolium chloride at 140 °C. The GVL remarkably enhanced the reaction rate and increased the xylose conversion and FF yield because the solvent effect of GVL could improve the catalytic activity of the acidic protons and suppress the side reactions. In addition, the ionic liquids showed satisfactory catalytic performance in the conversion of xylose to FF due to functionalization by introducing a SO3H group. The conversion of other C5 carbohydrates (arabinose and xylan) was also tested in this system, and moderate FF yields were achieved. Keywords:

xylose;

furfural;

SO3H-functionalized

ionic

liquid;

catalysis;

γ-valerolactone

1. Introduction Biomass is an abundant renewable resource in nature and can be converted into a variety of chemicals and advanced liquid fuels, which is very significant for the *

Corresponding author. Present address: State Key Laboratory of Clean Energy Utilization, Zhejiang University, Zheda Road 38, Hangzhou 310027, China. Tel.: +86 571 87952801; fax: +86 571 87951616. E-mail address: [email protected] (S. Wang).

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substitution of the dwindling fossil resources.1-3 Hemicellulose accounts for approximately 30% of all biomass, and pentose is an important component of hemicellulose.

4, 5

Furfural, a dehydration product of pentose, has been identified as

one of the most value-added bio-based platform chemicals because it can be used to produce a variety of chemicals and liquid fuels, such as furfuryl alcohol, γ-valerolactone (GVL), tetrahydrofuran, 2-methylfuran and alkyl levulinate.6-10 The current commercial technology for FF production mainly uses H2SO4 or HCl as a catalyst in the aqueous phase, which may cause equipment corrosion, and the resultant FF yield is limited due to undesired side reactions.11, 12 Therefore, it is necessary to develop a more advanced reaction system for FF production. The reaction medium is a key factor affecting the production of FF because the solvent effect highly influences the chemical thermodynamics of the reactions.13 Although water is a green and cheap solvent and has been used as a reaction solvent in the commercial production of FF, it easily causes side reactions due to its highly polar protonic property.12, 14 A recently developed biphasic solvent system consisting of water and an immiscible organic solvent can avoid the secondary reactions because the formed FF can be extracted into the immiscible organic solvent.15 However, the biphasic system requires extra organic solvent for extraction and usually needs the addition of some salt like NaCl to improve the extraction efficiency, increasing the complexity and cost of the system.16 More recently, monophasic systems based on renewable bio-based organic solvents have attracted intensive attention

17, 18

. In

particular, GVL, a typical compound that can be produced from FF, is recognized as a 5


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novel and promising solvent in FF production because it is a polar aprotic solvent and can promote the formation of FF.19, 20 Additionally, the boiling point of GVL (207 °C) is higher than that of furfural (162 °C), so the furfural can easily be recovered from the GVL by (vacuum) distillation.13 Ionic liquids, salts composed of organic cations and inorganic anions, are known as promising solvents due to their remarkable physicochemical properties.21, 22 The application of ionic liquids for biomass processing, including the production of FF, has attracted significant attention in recent years.21, 22 This is because ionic liquids have good chemical and thermal stability and desirable substrate-dissolving capacities, making them very suitable for the processing of biomass.23 Many studies have investigated the production of FF from xylose using ionic liquids as the reaction media and obtained FF yields as high as 80-90%.23 Nevertheless, the high cost and high viscosity of ionic liquids limit their application as reaction media on a large scale. It might be more reasonable to use the ionic liquids as both catalysts and co-solvents in small amounts rather than as the main solvents in large amounts. This can greatly reduce the dosage of ionic liquids and simplify the reaction system by avoiding the addition of extra catalysts, and thus improve the competitiveness of ionic liquids. On this base, an efficient system for FF production from raw biomass or polysaccharide could be expected by developing new ionic liquids that are able to dissolve substrates and simultaneously catalyze desired reactions. The production of FF from xylose is an acid-catalyzed conversion process. Many different acidic catalysts have been used for the production of FF, such as homogeneous organic acids and metal chlorides, 6


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heterogeneous acidic resins and zeolites.11, 24 Considering that ionic liquids may not have enough acidity to catalyze the reactions, it is desirable to functionalize them by introducing a SO3H group to enhance the acidity. SO3H-functionalized ionic liquids have been proven to be efficient in some acid-catalyzed reactions, including the hydrolysis of cellulose.25, 26 In this study, we investigated the conversion of xylose to FF in a novel system consisting of GVL solvent and a SO3H-functionalized ionic liquid catalyst. The effects of the solvent composition, SO3H-functionalized ionic liquid, and reaction parameters on the xylose conversion and FF yield were studied. The conversion of arabinose and xylan was also tested in this system.

2. Experimental 2.1 Materials Xylose, arabinose, FF and GVL were purchased from Aladdin Industrial Corporation. Xylan extracted from beech wood was purchased from Sigma Corporation.

The

SO3H-functionalized

ionic

liquids

1-propylsulfonic-3-

methylimidazolium chloride (IL1), 1-propylsulfonic-3-methylimidazolium hydrogen sulfate

(IL2),

1-butylsulfonic-3-methylimidazolium

chloride

(IL3)

and

1-butylsulfonic-3-methylimidazolium hydrogen sulfate (IL4) were purchased from Shanghai Chengjie Chemical Corporation. All of these chemicals were analytical grade and used without further purification. 2.2 Procedure for conversion of carbohydrates to FF In a typical experiment for FF production, 1 mmol sugar and 5 mL of solvent 7


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were charged into a glass pressure reactor (15 mL, Synthware) with a catalyst concentration of 0.1 M. The reactor was heated in an oil bath, and the magnetic stirring rate was maintained at 600 rpm. When the preset reaction temperature and time were reached, the glass reactor was taken out from the oil bath and was cooled by air flow. The solution was filtered through a 0.22-µm syringe filter prior to high-performance liquid chromatography (HPLC) analysis. Each experiment was repeated three times, and the resulting mean value and standard deviation are shown in the figures and table. 2.3 Analytical methods The filtered solution was analyzed on a Dionex HPLC system equipped with a Bio-Rad Aminex HPX-87H column and an RI 2000 refractive index detector. A H2SO4 solution (pH 2.5) was used as the mobile phase. The flow rate and column temperature were kept at 0.6 mL/min and 60 °C, respectively. The concentrations of xylose, arabinose and FF were determined by comparison against standard calibration curves. The retention times for furfural, xylose and arabinose are 9.34 min, 10.16 min and 43.65 min, respectively. The integration of the peaks was performed at a peak width of 5 and a slope of 70 by the N2000 HPLC chromatographic workstation. Typical HPLC chromatographs of the product solution after the conversion of xylose, arabinose and xylan are shown in Fig. S1 in the Supporting Information. The conversion of pentose (xylose and arabinose) and the yield of FF were calculated as follows: Pentose conversion=

moles of pentose reacted ×100% moles of starting pentose 8


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FF yield=

moles of FF produced ×100% moles of starting pentose

3. Results and discussion 3.1 Effect of solvent composition Fig. 1 shows the conversion of xylose to FF catalyzed by IL1 at different solvent compositions. The xylose conversion and FF yield increased with the reaction time in pure water at 140 °C. However, they were at very low levels, with values of 24.72% and 6.24%, respectively, at a reaction time of 240 min. This is in agreement with the finding that the activation energy for the conversion of xylose to FF is high (approximately 30 kcal/mol) in an acidic aqueous phase.27 Therefore, the production of FF from xylose in aqueous medium is always performed at a temperature higher than 150 °C.27 When the solvent composition was modified by adding GVL, the xylose conversion was significantly improved with the increasing concentration of GVL. At a reaction time of 60 min, the xylose conversion sharply increased from only 10.70% at a water-GVL ratio of 1:1 to 88.90% at a water-GVL ratio of 1:19, and they were 43.67% and 97.67%, respectively, when the reaction time was prolonged to 240 min. This indicates that the presence of GVL could enhance the reaction rate of xylose because its polar aprotic solvent effect could affect the stabilization of the acidic protons and decrease the activation energy relative to the protonated transition states.13, 28 The presence of GVL also affected the formation of FF. At a reaction time of 60 min, the yields of FF in water-GVL solvent at different ratios were approximately 2% (ratio 1:1), 7% (ratio 1:2), 21% (ratio 1:4) and 65% (ratio 1:19). This suggests that the solvent effect of GVL favored the formation of FF. The formed 9


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FF easily underwent secondary reactions in the highly polar protonic water, but they could be suppressed in aprotic GVL. Hu et al.29 analyzed the conversion of xylose in different solvents (water, alcohols, ketones, furan, hydrocarbon, ethers, phenols and esters) and found that the ester solvents were favorable for the selectivity of FF. At a water-GVL ratio of 1:19, the yield of FF increased and then decreased with the reaction time, and the maximum yield was 78.12% at 140°C for 180 min. This furfural yield was comparable to the previous results of approximately 80% from xylose in a similar GVL-based system catalyzed by H2SO4 at 160 °C for 60 min and by solid acid catalysts at 175 °C for 120 min.17, 18 This indicates that it is feasible to use an ionic liquid as a catalyst following sulfonation for furfural production from xylose. 3.2 Effect of reaction temperature As shown in Fig. 2, at 120 °C, the xylose conversion gradually increased from 50.28% for 60 min to 85.98% for 240 min. Accordingly, the yield of FF increased from 27.43% for 60 min to 59.73% for 210 min, and then it decreased due to secondary reactions. This suggests that a relatively high yield of FF could be achieved from xylose at low temperature in the reaction system. With the increasing reaction temperature, the xylose conversion increased to more than 90%, and the corresponding maximum yields of FF were 74.85% (130°C, 180 min), 78.12% (140°C, 180 min) and 72.82% (150°C, 90 min), indicating that increasing the temperature could shorten the time needed to reach the peak yield of FF. However, at high temperature and a long reaction time, the yield of FF diminished, and a small 10


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amount of the insoluble solid by-product humin was observed. The humin was formed from the condensation of sugar, FF and some dehydrated intermediates.30,

31

Nevertheless, the formation of humin in this system was much less than that in the pure aqueous phase, implying that the presence of GVL also clearly prevented the formation of the insoluble by-product. 3.3 Effects of catalyst and xylose dosage Fig. 3 presents the effects of the catalyst dosage on the xylose conversion and FF formation at 140 °C for 180 min. The conversion of xylose was only about 16% and the corresponding FF yield was nearly negligible when no acidic ionic liquid was added into the reaction system. In contrast, xylose conversion and FF yield significantly increased to about 89% and 63% respectively at a low IL1 concentration (0.02 M). With IL1 concentration increasing, the xylose conversions exceeded 95%, and the yields of FF were 69.46%, 78.12% and 70.88% at 0.04 M, 0.1 M and 0.2 M, respectively. For further assessment of the effect of ionic liquid at similar xylose conversion, the reaction times for the cases at 0 and 0.02 M IL1 concentration were prolonged. It cost 18 h to reach a xylose conversion of over 95% with a very low FF yield of less than 3% without adding IL1, while it took 4h to obtain a xylose conversion of about 95% and a FF yield of 65.31% at 0.02 M IL1 concentration. These suggest the good catalytic performance of the SO3H-functionalized ionic liquid for the conversion of xylose to FF, and indicate the feasibility of developing a simplified reaction system using a small amount of ionic liquid without adding extra catalyst. A high concentration of acidic protons could promote the occurrence of side 11


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reactions such as the Diels–Alder reaction, which can lead to a loss of formed FF.32 This is because the oxygen atom of the aldehyde group in FF could form hydrogen bonds with the surrounding water molecules, which significantly increases the dipole moment of the FF molecule.33 This interaction between the oxygen atom and the water molecule is enhanced when the water molecule is protonated in acidic solution, and the electron density in the aldehyde group thus became higher while the electron density in the furan ring became lower, which greatly favored the Diels-Alder reaction.33 The use of highly acidic solutions could also lead to the hydrolytic fission of the aldehyde group to form formic acid.12, 33 From a practical point of view, a higher initial xylose load was good for the economy of this technology. As displayed in Fig. 4, the initial xylose load had only a weak effect on the xylose conversion, but it showed an obvious impact on the yield of FF. When the initial xylose load increased from 30 mg/ml to 300 mg/ml, the corresponding yield of FF decreased from 78.12% to 46.41%, suggesting that a higher xylose concentration could lead to a loss of FF. This was consistent with the results of Danon et al.34, who analyzed the decomposition kinetics of FF in a xylose solution and found that the presence of xylose increased the reaction rate of the FF decomposition. A substantial amount of insoluble humin was also observed at a high initial xylose load, which resulted from the large number of active intermediates formed from the dehydration of xylose that promoted condensation and polymerization. 3.4 Effects of acidic ionic liquids 12


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As presented in Fig. 5, all four acidic ionic liquids showed good catalytic performance in the conversion of xylose, with conversions as high as 95%. As for the FF formation, these acidic ionic liquids had varying activities, and the yields of FF were 78.12% (IL1), 70.90% (IL2), 72.36% (IL3) and 69.49% (IL4). In contrast, the xylose conversion and FF yield were neglectable without the addition of any catalyst under comparable conditions. This indicates that the SO3H-functionalized ionic liquids could efficiently catalyze the conversion of xylose to FF. Fig. 6 shows the proposed mechanism of the conversion of xylose to FF catalyzed by the SO3H-functionalized ionic liquid IL1. During the formation of FF, the xylose mainly underwent several steps including ring-opening, enolization and dehydration.35 The SO3H group of the cation in the ionic liquid acted as a Brønsted acid that donated protons, and it also functioned as a conjugate base that accepted protons, which could catalyze the enolization of xylose and the subsequent dehydration of the intermediate to FF.36 Meanwhile, the chloride anion could interact with the substrate and acidic proton in the imidazolium ring through hydrogen bond interactions, which provided a polar environment along with the imidazolium cation to stabilize the intermediates and transition states, favoring the selective formation of FF.36 Therefore, the functionalization of ionic liquids by introducing a SO3H group could be a useful strategy in the preparation of catalysts for the production of FF. 3.5 Conversion of other C5 carbohydrates The conversion of arabinose and xylan to FF in the reaction system was also tested. Arabinose is the second most abundant pentose in biomass after xylose, but 13


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there have been relatively few studies on its conversion to FF.11 As shown in Table 1, IL1 had good catalytic activity for the arabinose conversion. At 140°C and 150°C, the arabinose achieved a conversion of more than 90%. However, the corresponding yields of FF were 20-25%, much lower than that from xylose under the same reaction conditions. Gallo et al.18 also found that the yield of FF from arabinose was approximately half of that from xylose when they performed the conversion of pentose in H2SO4 solution. Garrett et al.37 analyzed the kinetics of the conversion of arabinose and xylose to FF and found that the reaction rate of xylose to FF was much higher than that of arabinose to FF. They proposed that the steric positioning of the hydroxyls in the pentose was the controlling factor leading to different reactivities. Dussan et al.38 further suggested that the distribution of isomers of pentose was another factor affecting the reactivity. The predominant form of xylose was the β-pyranose form, whereas for arabinose, the α-pyranose form was predominant. The α-pyranose tautomer was more stable than β-pyranose and was difficult to dehydrate to FF, so the corresponding selectivity towards FF was lower.38 Xylan is a polysaccharide consisting of β-1,4-xylose, and it must be hydrolyzed to xylose prior to being dehydrated to FF.39 As shown in Table 1, the xylose yield from the hydrolysis of xylan catalyzed by IL1 was 21.63% at 130 °C, and it decreased to 6.39% at 150 °C. Meanwhile, the yield of FF increased from 17.27% at 130 °C to 43.91% at 150 °C, suggesting that IL1 could catalyze the conversion of xylan to FF under moderate conditions. The above results showed that using SO3H-functionalized ionic liquids as 14


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catalysts had good potential in furfural production. For the practical application of ionic liquids, the recovery of ionic liquids would be another important issue due to their unique physicochemical properties such as high viscosity and low volatility.40 In recent years, many methods are developed to recover ionic liquids, including distillation, extraction, adsorption, induced phase separation, membrane-based methods, etc.40,

41

Nevertheless, the recovery of ionic liquids is still a challenge

because these methods have some disadvantages to overcome. 41, 42 Additionally, a common criterion for the recovery of ionic liquids from specific reaction system is currently absent. Therefore, systematic investigation will be performed in the future study to determine a feasible and efficient method for recovering the ionic liquids used in this study.

4. Conclusion In this study, an efficient and green system was developed for FF production from xylose in GVL using a SO3H-functionalized ionic liquid as a catalyst. A high FF yield of 78.12% was obtained from xylose at 140°C. The introduction of GVL into the solvent system could significantly increase the reaction rate and suppress the secondary reactions of the formed FF. The SO3H-functionalized ionic liquids showed good performance in the conversion of xylose to FF. Moderate FF yields were also obtained from arabinose and xylan, suggesting that this system has good potential for FF production from C5 carbohydrates under mild conditions.

Acknowledgements The authors are grateful for the financial support from the National Natural 15


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Science Foundation of China (51476142), the National Science and Technology Supporting Plan Through Contract (2015BAD15B06) and the Guangdong Province Key Laboratory of Efficient and Clean Energy Utilization (2013A061401005).

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Figure captions Fig. 1 Effect of solvent composition on xylose conversion (a) and FF yield (b) (140 °C, 0.1 M IL1, 30 mg/ml xylose) Fig. 2 Effect of reaction temperature on xylose conversion (a) and FF yield (b) (water-GVL (1:19), 0.1 M IL1, 30 mg/ml xylose) Fig. 3 Effect of catalyst dosage on xylose conversion and FF yield (140 °C, water-GVL (1:19), 140min, 30 mg/ml xylose) Fig. 4 Effect of initial xylose load on xylose conversion and FF yield (140 °C, 180 min, water-GVL (1:19), 0.1 M IL1) Fig. 5 Effect of various acidic ionic liquids on xylose conversion and FF yield (140 °C, 180 min, water-GVL (1:19), 30 mg/ml xylose 0.1 M IL1) Fig. 6 Proposed mechanism of the conversion of xylose to FF catalyzed by IL1.

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Fig. 1

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Fig. 2

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Fig. 3

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Fig. 4

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Fig. 5

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Fig. 6

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Table 1 conversion of arabinose and xylan in the reaction system of GVL and SO3H-functionalized ionic liquid (30 mg/ml arabinose or xylan 0.1 M IL1)

a

Feedstock

Temperature

Time

Conversion

Furfural yield

Xylose yield

arabinose

130

180

84.92 (±0.38)a

21.24 (±0.62)

-

arabinose

140

180

92.69 (±0.62)

24.46 (±0.51)

-

arabinose

150

180

97.15 (±0.58)

23.38 (±0.71)

-

xylan

130

180

-

17.27 (±0.53)

21.62 (±0.35)

xylan

140

180

-

29.26 (±0.44)

13.87 (±0.51)

xylan

150

180

-

43.91 (±0.85)

6.39 (±0.24)

Standard deviation

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Conversion of C5 Carbohydrates into Furfural ... - ACS Publications

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